5 Answers
5

The seat tube is split because it has rear suspension - the rear of the bike rotates around a pivot located just in front of the bottom bracket. Several early full-suspension bikes used a similar design for the rear pivot; this design was then knocked off by the lower-end manufacturers looking to cash in on the popularity of mountain bikes. It's extremely simple and gives the bike the expensive "full-suspension" look. Unfortunately it mostly just adds weight, pedal bob, and complexity to an already heavy turd of a bike.

Most modern full-suspension bikes use more expensive rear linkages and advanced suspension designs, so this older design has almost entirely been relegated to department store bikes.

That's the cheapest, easiest way to make a rear suspended bike. It's a combination of a simple, almost two dimensional frame which is easy to design and manufacture, and the open layout making it easy to assemble. On a cheap bike those two factors outweigh anything else.

The main problem with the design is torsional rigidity along the frame. Because there are no seat stays the rear wheel twists easily and so does the seat. To compensate for that the "ẗop tube" (that holds the seat) and "chain stays" (that hold the rear wheel) are made heavier than they are on an unsuspended bike. The design pictured attempts to minimise this by using triangulated stays for the wheel, but all the torsion still goes onto the pivot resulting in poor performance. Most noticeable when standing up to pedal hard, especially if you tilt the bike (think BMX racing).

Notice that in the picture above all the main frame tubes are oversize, and most have non-circular profiles. This is an attempt to maximise strength in the directions it's most needed without compromising the budget by using harder steel (which is more expensive, but more importantly requires much more expensive tooling).

Worse, some manufacturers use the same design for a rigid rear end, giving you all the disadvantages of both.

Here is an example of high-end carbon fiber frame with similar geometry = Trek Y Foil. It proved to be unsuccesful design, but low-end bicycle manufacturers copied it anyway (also using cheaper materials and simplified design).

It is clear that this design is heavy, weakens the structural integrity of what was once a simple diamond frame, introduces bushes that wear, does the 'pedal bob' and complicates the maintenance. However, what is in it for the manufacturer?

'Horst Link' and other properly thought out suspension designs are patented, these patents cost money to licence, whereas the design discussed here is royalty free. It costs nothing apart from a few lumps of ugly metal.

Not wishing to defend the design, however, it does have some benefits with 24 and 20 inch wheel children's bikes, providing a lower standover height and providing some protection for the back wheel from kerbs and other objects 'in the playground'.

From a retailers perspective the differential between the non suspended and cheap suspended variant can be all of £10, which is £10 more in the till. The customer gets 'full suspension' for all of £10, which can be sold as a 'bargain'...